This invention relates to a digital amplifier and in particular to a protection circuit when output is short-circuited.
FIG. 2 is a circuit diagram to show a configuration example of a digital amplifier in a related art. In FIG. 2, T denotes a transformer, D3 and D4 denote rectification elements (diodes), and C1 and C2 denote smoothing capacitors. The transformer T, the rectification elements D3 and D4, and the smoothing capacitors C1 and C2 make up a positive-negative power supply of capacitor input type. On the other hand, in a digital amplifier section, SS1 and SW2 denote switching transistors such as MOSFETs driven by output of a PWM modulation circuit (not shown), D1 and D2 denote diodes (flywheel diodes) connected to the switching transistors SS1 and SW2 in parallel, LF denotes a coil, CF codes a capacitor, RL denotes a load (loudspeaker), and SPOUT denotes a loudspeaker output terminal.
In the digital amplifier in FIG. 2, the switching transistors SS1 and SW2 are driven complementarily by a signal provided by performing PWM modulation of an input signal, and the load RL is driven through a low-pass filter made up of the coil LF and the capacitor CF. That is, as the switching transistor SS1 is brought into conduction (the switching transistor SW2 is brought out of conduction), current I+ flows from the positive power supply and as the switching transistor SW2 is brought into conduction (the switching transistor SS1 is brought out of conduction), current I− flows into the negative power supply, whereby the load RL is driven, as shown in FIG. 2. Such a digital amplifier is known as a very highly efficient amplifier.
By the way, an amplifier is provided with a protection circuit to protect a product against an abnormal state of a short circuit of loudspeaker output. Generally, a protection circuit of a digital amplifier is made up of an overcurrent detection circuit LDT on the negative power supply side (which will be hereinafter called low side) and an overcurrent detection circuit HDT on the positive power supply side (which will be hereinafter called high side), as shown in FIG. 3. The overcurrent detection circuit LDT is made up of resistors R1 and R2 and an npn transistor Q1 for detecting an overcurrent when loudspeaker output is short-circuited by observing the voltage of the resistor R1 inserted into the switching transistor SW2 on the low side in series and turning on the transistor Q1 at the overcurrent detection time. Likewise, the overcurrent detection circuit HDT is made up of resistors R3 to R5 and a pnp transistor Q2 for observing the voltage of the resistor R3 inserted into the switching transistor SS1 on the high side in series and turning on the transistor Q2 at the overcurrent detection time, thereby turning on the transistor Q1. When the transistor Q1 is turned on, a control circuit (not shown) performs the protection operation of stopping the switching operation, turning off the power, etc.
However, the protection circuit as in FIG. 3 needs to detect a current at one instantaneous large output time in a normal state as a non-overcurrent; on the other hand, when an overcurrent is caused by a short circuit of loudspeaker output, the protection circuit is required to promptly detect the overcurrent. Thus, it is difficult to set the operation point; this is a problem.
In the half-bridge digital amplifier as in FIG. 2 or FIG. 3, it is known that a power pumping phenomenon in which the voltages of the positive and negative power supplies is placed out of balance is known. The power pumping phenomenon will be discussed with FIG. 4. FIG. 4 is a drawing to show the voltages and currents of the parts when positive voltage VRL is supplied to the load RL in the digital amplifier in FIG. 2.
As shown in (A) of FIG. 4, when the positive voltage VRL is applied to the load RL, time period T1 during which the switching transistor SS1 conducts becomes longer than time period T2 during which the switching transistor SW2 conducts. In the time period T1 from time t1 to time t2, a current I1 flows on a path from the positive power supply to the switching transistor SW1, the coil LF, the load RL, and ground ((C) of FIG. 4). Next, at time t2, the switching transistor SW1 is brought out of conduction and the switching transistor SW2 conducts. Accordingly, voltage Vo at the connection point of the switching transistors SS1 and SW2 changes from +V to −V. On the other hand, since the coil LF of an inductive load exists, the current changes with a delay from the change in the voltage and flows in the opposite direction to the direction of the voltage for one time. That is, a current I2 continues to flow through a path of the diode D2, the coil LF, the load RL, the capacitor C2 ((D) of FIG. 4). Since the switching frequency of the PWM modulation circuit for driving the switching transistor SW1, SW2 is a very high frequency of several hundred kHz, for example, time t3 is reached before current I− flows into the negative power supply through the switching transistor SW2, and the switching transistor SS1 is brought into conduction and the switching transistor SW2 is brought out of conduction and again the current I1 flows. Thus, load current IL shown in (B) of FIG. 4 flows into the load RL.
The direction of the current I2 is opposite to the direction of the current I− which should essentially flow at the conduction time of the switching transistor SW2, and the capacitor C2 on the low side is charged. Thus, voltage V2 charging the capacitor C2 becomes higher than voltage V1 across the capacitor C1 on the high side (V2>V1).
In contrast, when a negative voltage is applied to the load RL, the operation opposite to that described above is performed; the voltage V1 charging the capacitor C1 on the high side becomes higher than the voltage V2 across the capacitor C2 on the low side (V1>V2).
As the power pumping phenomenon as described above occurs, the voltages of the positive and negative power supplies is placed out of balance and the operation efficiency is degraded and there is a possibility that the amplifier may be destroyed due to overvoltage. The power pumping phenomenon becomes a more noticeable problem when loudspeaker output is short-circuited. In the digital amplifier in the related art, if the protection circuit shown in FIG. 3 operates, the overcurrent caused by short-circuiting of the loudspeaker output can be detected and consequently the power pumping phenomenon can be prevented from destroying the amplifier. However, it is difficult to set the operation point of the protection circuit as described above and it may be impossible to promptly find occurrence of an overcurrent and consequently there is a possibility that the amplifier may be destroyed due to the overvoltage caused by the power pumping phenomenon.
On the other hand, as another example of the protection circuit, a protection circuit having a current detection coil provided for a low-pass filter of loudspeaker output for detecting an overcurrent flowing into the coil of the low-pass filter is proposed. (For example, refer to JP-A-5-160649) FIG. 5 is a circuit diagram to show the configuration of the protection circuit disclosed in JP-A-5-160649. Components similar to those previously described with reference to FIG. 2 are denoted by the same reference numerals in FIG. 5. The protection circuit is made up of a current detection coil 1, a rectifier 2 an attenuator 3, a capacitor 4, and an R/S latch circuit 5. The current detection coil 1 wound around coil LF of low-pass filter detects a current flowing through the coil LF by mutual induction action and outputs the detected current as a detection signal. The detection signal is half-wave or full-wave rectified by the rectifier 2 and then the level is attenuated by the attenuator 3. If the level of the input detection signal becomes a predetermined value or more, the R/S latch circuit 5 outputs a latch signal indicating occurrence of an overcurrent. Thus, the protection circuit detects an overcurrent when loudspeaker output is short-circuited.
However, the protection circuit in FIG. 5 involves problems of complicated configuration and high cost. To detect the current flowing through the coil LF of the low-pass filter using the current detection coil 1 with good accuracy, it is desirable that the voltage across the coil LF should be increased, and the resistance component of the coil LF needs to be made large. However, making large the resistance component inserted in series with the load RL is undesirable from the viewpoint of driving the load RL (loudspeaker). Thus, in the protection circuit in FIG. 5, the accuracy of current detection cannot be enhanced and it may be impossible to promptly find occurrence of an overcurrent and there is a possibility that the amplifier may be destroyed due to the overvoltage caused by a power pumping phenomenon.
The applicant has not found any related art documents relevant to the invention before the application time except the related art document determined in the related art document information described in the specification.
As described above, in the protection circuits shown in FIGS. 3 and 5, there is a possibility that occurrence of an overcurrent caused by short-circuiting of loudspeaker output cannot promptly be detected, and there is a possibility that the amplifier may be destroyed due to a power pumping phenomenon which becomes noticeable as loudspeaker output is short-circuited.